Communication of information over a power line is useful in a number of situations. In order to do so, individual transmitters and receivers associated with them may be placed at varying locations along a power line to send and receive data. For example, it may be desired to transmit power consumption data from each of the users to a central station. In other examples, it may be desired to send signals from a central location to individual users for load control or other applications.
A number of systems have been proposed for communicating over AC power lines in the past. All of the systems must cope with the fact that AC power lines are usually noisy and are already connected to various control and other devices capable of affecting the proper operation of a communication system. It is also necessary to have a system that allows for the simultaneous communication of a plurality of signals to and from a multitude of users.
In some power line communication systems an input signal is converted to a frequency by a voltage to frequency converter and the output of the voltage to frequency converter is used by an FM transmitter to modulate a carrier signal which is then transmitted over the power line. Such a system requires two stages of modulation. Such a system may provide a 1 to 6 KHz varying frequency at the V/F converter and a carrier of about 200 KHz at the output of the FM transmitter. Such high frequency carrier signals are not capable of passing through power line transformers and thereby are limited in their utility for applications requiring communication between a central station and users through transformers.
In other power line communications systems, two stages of modulation are provided to a signal provided by a data source having a very low bandwidth but the carrier frequency provided by the second stage of modulation is less than 10 KHz. Such systems however, have still required two modulation steps to achieve the signal to be transmitted on the power line because the previous modulation techniques employed were incapable of producing, through a single modulation step, narrow bandwidth carriers at higher frequencies.
To provide a reliable communication system, for transmission of very low bandwidth information, there is a need for a transmission system where a single modulation step provides a highly stable low bandwidth communication signal suitable for communication via power lines carrying AC power.